The present invention relates to an automatic compensation system for use in MODEM, and in particular to a system for automatically compensating the transmitting (outgoing) signal level by detecting the receiving (incoming) signal level from a communication channel.
Generally, in performing data communication between remote places by connecting a communication system such as MODEM to a public switching telephone network (PSTN), a data access arrangement (DAA) that meets a predetermined standard should be used therein. Thus, a MODEM for executing the data communication via the public switching telephone network should be operated with the transmitting level of CCITT regulation or Bell regulation. The CCITT or Bell regulation recommends to transmit with the signal level of -10 dBm .+-.1 dBm, considering therein the gain attenuation (-33 dBm) in transmission lines, while it recommends that the receiving side of the MODEM should receive and demodulate a receiving signal having the gain level of at least -43 dBm.
In most existing MODEMs, the transmitting gain thereof is usually fixed to a constant value of the transmitting gain of -10 dBm .+-.1 dBm considering only the gain attenuation (-33 dBm) of transmission line on which data is transmitted. However, in a MODEM that transmits data to the public switching telephone network by setting its transmitting gain to -10dBm.+-.1dBm as aforementioned, the following problems arise. That is to say, there has been a problem that the possibilities of the interference of data and the transmitting and receiving error occurrence are considerably high, because the factors resulting from the increase or decrease of the gain attenuation of the transmission line have been not taken into account. Inevitably, the gain attenuation of a transmission line increases or decreases depending upon the variation of line conditions according to the weather and environment, and the difference of local conditions of the public switching telephone line according to the area. Therefore, a problem can be rendered that the data is not exactly received at a remote side MODEM, according to the extent of gain attenuation in the public switching telephone trasmission line, if the transmitting gain is always set with a constant level.